We investigate the influence of Na on the growth of Cu(In,Ga)Se-2 thin-films by three model experiments. First, we examine the influence of Na on the Se activity during selenisation of Mo films on soda-lime and borosilicate glass after growth and after thermal treatments. Second, we analyse the location and possible binding partners of Na in polycrystalline Cu(ln,Ga)Se2 prior to and after air-exposure. The final experiment focuses on the identification of the chemical state of Na on the surface of as grown and air-exposed films. Our experiments demonstrate that Na influences the growth of CIGS Cu(ln,Ga)Se-2 due to its interaction with Se. in non-air-exposed films Na is mainly localised in the form of sodium-polyselenides (Na2Sex) at the grain boundaries. We conclude that Na2Sex acts as Se-reservoir during film formation and oxidation.